1. Field of the Invention
The present invention relates to a process for the preparation of purified xanthan gum, and more particularly to a process for the improvement in transparency of fermented broth and aqueous solution of xanthan gum by enzymatic treatment.
2. Description of the Related Art
Xanthan gum can be obtained by a well-known fermentation process. It is recovered by precipitation with isopropanol from a fermented broth of a xanthan gum-producing bacterium, e.g., Xanthomonas campestris a species in the genus Xanthomonas. (This compound and process for preparing it are described in U.S. Pat. No. 3,659,026, col. 4.)
Other processes for the manufacture of xanthan gum use, in place of Xanthomonas campestris, such other known xanthan gum-producing bacteria as Xanthomonas carotate, incanae, begoniae, papavericola, translucens, vasculorum, and hederae. All these species can produce a xanthan gum fermented broth.
The broth at the end of fermentation contains from 0.5 to 2 percent by weight of undissolved matter including unconsumed nutrients and bacterial cell residues, besides about 2 to 5 percent by weight of xanthan gum. Naturally, an aqueous solution of solid xanthan gum separated by extraction from the broth has very low transparency. This presents problems in the fields where clear products are required, such as food and cosmetic industries, and in applications for secondary recovery of petroleum.
For the purification of xanthan gum, methods that depend on centrifugation or cake filtration for the removal of the undissolved matter from the broth are commonly known. In either case, however, the broth is so viscous that it needs dilution with water and concentration, and the cost and operation of these additional steps make the methods impractical. A variation of those methods suggests heating of the broth to enhance its filtration properties.
One effective approach is solubilizing the undissolved matter in the broth by enzymatic treatment. Many proposals have hitherto been made in this direction.
For example, U.S. patent application Ser. No. 449,875 of 1974 (Japanese Patent Provisional Publication (Kokai) No. 50-121493) and U.S. patent applications Ser. Nos. 513,810/1974, 3,966,618/1976, 4,010,071/1977, and 416,525/1979 propose clarification by use of alkaline protease and neutral protease. They report in fact, that xanthan gum solutions cannot necessarily be made quartz-crystalline clear and there remains certain turbidity in the solutions. Thorough transparency is yet to be attained.
British Patent Application No. 8,132,564/1981 (Japanese Patent Kokai No. 58-81792) teaches the use of acidic protease and neutral protease. U.S. patent application Ser. No. 797,093/1977 (Japanese Patent Kokai No. 62-44918) teaches bringing a solution that results from protease treatment into contact with a siliceous solid matter and then removing cell bodies from an aqueous polymer solution. French Patent Application No. 8,110,403/1981 (Japanese Patent Kokai No. 57-202303) proposes an enzymatic treatment using polysaccharase and protease. U.S. Pat. No. 4,431,734 (Japanese Patent Kokai No. 63-287494) teaches combined use of a polygalacturonase-active enzyme and a protease-active enzyme. U.S. patent application Ser. No. 147,812/1980 (Japanese Patent Kokai No. 57-5698) proposes the adoption of a composite enzyme having both .beta.-1,3-glucanase- and protease-activities. None of the proposed treatments with protease have however, proved satisfactorily effective.
Among other enzymatic processes so far proposed are the utilization of a nuclease-active enzyme according to British Patent Application No. 8,431,653/1984 (Japanese Patent Kokai No. 61-146193) and the purification by the action of cellulase according to French Patent Application No. 8,021,395/1980 (Japanese Patent Kokai No. 57-91194). They too have failed to be adequately effective.
Japanese Patent No. 1318520 (Japanese Patent Kokai No. 60-44919) introduces a treatment with the simultaneous addition of lysozyme, N-acetylmuramyl-L-alanine amidase, and peptidase. These enzymes are known to be able to lyse cell walls, but they have only slight action directly on Gram-negative bacteria. Despite long treatment time required, their effect is disappointingly low.
As summarized above, there have been known methods for removing water-insoluble microbial residues and culture medium-derived undissolved matter, including the methods relying upon centrifugation or cake filtration or upon enzymatic treatment to make the undissolved matter soluble in water. With those methods, however, it has been difficult to obtain a xanthan gum fermented broth with a high degree of transparency.